Which Resins Require Carbide Gate Bushings? Wear Resistance Guide

The Wear Mechanism

Gate bushing wear occurs at the gate orifice — the narrowest point in the flow path where resin velocity is highest. As molten resin passes through the gate at 200–1,000 mm/s, any solid particles suspended in the melt act as abrasive media that progressively erode the gate surface.

The wear rate depends on four factors:

• Filler hardness — Glass fibers (Mohs 6.5) cause more wear than calcium carbonate (Mohs 3). Carbon fibers create cutting-action wear despite lower hardness.
• Filler content — Wear rate increases roughly proportionally with filler percentage. 30% GF wears approximately 2× faster than 15% GF.
• Injection speed — Higher injection speed means higher particle velocity through the gate, increasing erosive energy.
• Gate diameter — Smaller gates concentrate the flow into a smaller area, increasing local velocity and wear rate.

According to tribology research documented in ScienceDirect's abrasive wear reference, the volume of material removed by abrasive wear is proportional to the applied load, sliding distance, and inversely proportional to the hardness of the wearing surface — which explains why carbide (1,400 HV) wears so much slower than steel (500 HV).

Resin-by-Resin Wear Classification

*PVC life is limited by corrosion, not abrasive wear. Steel bushings pit and corrode from hydrochloric acid gas released during processing.

Carbon Fiber: A Special Case

Carbon fiber-filled resins deserve special attention. Although individual carbon fibers are softer than glass fibers, they cause faster gate wear for two reasons:

• Brittle fracture — Carbon fibers break into sharp, angular fragments during plasticization. These fragments have cutting edges that create groove-type wear, whereas glass fibers tend to create smoother abrasive wear.
• Higher fiber stiffness — Carbon fiber's elastic modulus (230–400 GPa) is 3–5× higher than glass fiber (70–85 GPa). Stiffer fibers exert more contact pressure on the gate surface.

Industry data shows that 20% carbon fiber causes approximately the same gate wear rate as 30% glass fiber. For carbon fiber compounds above 15%, carbide gate bushings are always recommended. According to JIS K 7075 testing standards for carbon fiber composites, fiber fragment size after processing varies with screw design and back pressure, both of which affect downstream gate wear rates.

Beyond Wear: Corrosion Resistance

Some resins damage gate bushings through chemical attack rather than mechanical wear:

• PVC — Releases hydrochloric acid (HCl) gas at processing temperatures. HCl corrodes standard tool steel, causing pitting at the gate orifice. Carbide is chemically inert to HCl.
• POM (acetal) — Generates formaldehyde gas, which can corrode steel at elevated temperatures over extended production runs.
• Flame-retardant PA/PBT — Brominated and phosphorus-based flame retardants decompose into corrosive compounds during processing.

For these resins, the choice is between carbide (best corrosion resistance) and stainless steel (moderate corrosion resistance at lower cost). If the resin is both corrosive and filled (e.g., PBT-GF30-FR), carbide is the only viable option.